1. Field of the Invention
The present invention relates to a rotation angle sensor that includes a rotating magnet and a magnetic sensor for detecting a magnetic field produced by the magnet, the rotation angle sensor detecting the rotation angle of the magnet.
2. Description of the Related Art
In recent years, magnetic rotation angle sensors have been widely employed to detect the rotational position of an object for various types of use such as for detecting the rotational position of the steering of automobiles. For example, JP-A-2007-93280 and JP-A-2010-66196 disclose a magnetic rotation angle sensor that includes a rotating magnet and a magnetic sensor for detecting a magnetic field produced by the magnet. The magnetic rotation angle sensor detects the rotation angle of the magnet on the basis of the detection output from the magnetic sensor.
In the rotation angle sensor that includes a rotating magnet and a magnetic sensor, for example, the magnet has an end face perpendicular to the rotation axis and has a magnetization in a direction perpendicular to the rotation axis, while the magnetic sensor is disposed to face the end face of the magnet with a predetermined spacing therebetween. The performances required of the rotation angle sensor having such a configuration are that the magnetic field that is produced by the magnet and applied to the magnetic sensor should be high in strength and that a difference between the actual rotation angle of the magnet and the rotation angle of the magnet detected on the basis of the detection output from the magnetic sensor should be small. The rotation angle of the magnet detected on the basis of the detection output from the magnetic sensor will hereinafter be referred to as the detection angle. The difference between the actual rotation angle and the detection angle will hereinafter be referred to as the angle error.
The magnetic sensor may be subjected not only to the magnetic field produced by the magnet but also to another magnetic field such as a leakage magnetic field from a motor or the earth's magnetic field. The requirement that the magnetic field that is produced by the magnet and applied to the magnetic sensor should be high in strength is inevitable to relatively reduce the effects of the magnetic fields to be applied to the magnetic sensor other than the magnetic field that is produced by the magnet. Increasing the strength of the magnetic field that is produced by the magnet and applied to the magnetic sensor can be achieved by, for example, forming the magnet from a magnetic material that has a high residual flux density or increasing the size of the magnet.
On the other hand, one cause of the angle error is a misalignment between the magnet and the magnetic sensor. The positions of the magnet and the magnetic sensor relative to each other may slightly deviate from their desired positions during the fabrication of the rotation angle sensor or during use of the rotation angle sensor. It is assumed here that on a virtual plane parallel to the end face of the magnet, the direction of the magnetic field produced by the magnet is parallel to the direction of magnetization of the magnet when seen at the position at which the rotation axis intersects the virtual plane. The aforementioned position will hereinafter be referred to as the center position. On the virtual plane, the position of the magnetic sensor at which the rotation axis passes through the center of the magnetic sensor is assumed to be a desired position of the magnetic sensor. At a position away from the center position on the aforementioned virtual plane, the direction of the magnetic field produced by the magnet may be different from that at the center position. Accordingly, when the position of the magnetic sensor deviates from the desired position on the aforementioned virtual plane, the direction of the magnetic field detected by the magnetic sensor may be different from that detected when the magnetic sensor is located at the desired position. An angle error can thus occur due to a misalignment between the magnet and the magnetic sensor. The rotation angle sensor is required to have a small angle error even in the presence of a misalignment between the magnet and the magnetic sensor.
JP-A-2007-93280 discloses a technology for reducing the angle error for a rotation angle sensor that has a disc magnet and a magneto-electric transducer, by disposing the magneto-electric transducer at such a position that the angle error resulting from a misalignment of the rotation axis of the disc magnet is reduced.
JP-A-2010-66196 discloses a technology for reducing the angle error by devising the shape of the magnet and the arrangement of the magnetic detector section. In the technology, the magnet has a proximal portion around the center axis of rotation, a first outer peripheral portion outside the proximal portion, and a second outer peripheral portion outside the first outer peripheral portion. The thickness of the proximal portion in a direction parallel to the center axis of rotation is greater than the thickness of each of the first outer peripheral portion and the second outer peripheral portion in the direction parallel to the center axis of rotation.
For a rotation angle sensor that includes a rotating magnet and a magnetic sensor, as described above, it is required that the magnetic field produced by the magnet and applied to the magnetic sensor should be high in strength and that the angle error resulting from a misalignment between the magnet and the magnetic sensor should be small. To meet these requirements, a possible approach is to increase the strength of the magnetic field produced by the magnet and locate the magnetic sensor at such a position that the angle error resulting from a misalignment between the magnet and the magnetic sensor is reduced.
The inventors of this application have found by simulation that in the vicinity of the end face of a cylindrical magnet, there exists a region in which the direction of the magnetic field is substantially the same as the direction of the magnetic field at the desired position. Such a region will hereinafter be referred to as the parallel field region. The inventors have also found that the area of the parallel field region on a virtual plane parallel to the end face of the magnet varies depending on the distance between the end face of the magnet and the virtual plane. It can thus be thought that locating the magnetic sensor on a virtual plane that maximizes the area of the parallel field region would reduce the angle error resulting from a misalignment between the magnet and the magnetic sensor.
On the other hand, to increase the strength of the magnetic field produced by a cylindrical magnet, increasing the magnet in thickness (dimension in the direction of the rotation axis) is effective. However, it has been found that increasing the magnet in thickness would cause the following problem. That is, it has been found by simulation that increasing the magnet in thickness reduces the distance between the end face of the magnet and the virtual plane that maximizes the area of the parallel field region. Accordingly, if the magnet is increased in thickness and the magnetic sensor is located on the virtual plane that maximizes the area of the parallel field region, the distance between the end face of the magnet and the magnetic sensor may be excessively reduced to cause the magnetic sensor to contact and damage the magnet.
As such, in the case of using a cylindrical magnet, it is difficult to increase the strength of the magnetic field produced by the magnet and applied to the magnetic sensor and reduce the angle error resulting from a misalignment between the magnet and the magnetic sensor without causing an excessive decrease in the distance between the magnet and the magnetic sensor. The technologies disclosed in JP-A-2007-93280 and JP-A-2010-66196 could not solve the problem.